In general, a hydraulic excavator as a representative example of construction machines is configured with an automotive lower traveling structure, an upper revolving structure that is mounted on the lower traveling structure to be capable of revolving thereon and a front device that is mounted on the upper revolving structure to be capable of tilting and lifting thereto. The front device includes a boom, an arm, a bucket and a plurality of hydraulic actuators composed of a boom cylinder, an arm cylinder and a bucket cylinder that respectively drive these above components, and operates each of the hydraulic actuators, thereby performing an excavating work of earth and sand.
The upper revolving structure is formed as a support structure for being mounted on the lower traveling structure. The upper revolving structure is provided with a revolving frame on which the front device is mounted in the front side to the support structure, a counterweight that is provided in the rear end to the revolving frame for acting as a weight balance to the front device, an engine that is positioned in the front side to the counterweight and is mounted in the rear part of the revolving frame in a horizontal state of extending in the left-right direction, an exhaust gas purifying device that is provided in an exhaust pipe of the engine with a urea selective reduction catalyst removing nitrogen oxides in an exhaust gas, a urea water injection valve that is provided in the exhaust gas purifying device and injects urea water as a reducing agent to the upstream side of the urea selective reduction catalyst, a urea water tank that reserves therein the urea water to be supplied to the urea water injection valve and a urea water pump that delivers the urea water reserved in the urea water tank to the urea water injection valve.
In this case, a suction conduit line, in which the urea water to be suctioned from the urea water tank toward the urea water pump flows, establishes a connection between the urea water tank and the urea water pump. A return conduit line, in which the urea water to be returned from the urea water pump toward the urea water tank flows, establishes a connection between the urea water pump and the urea water tank. Further, a supply conduit line, in which the urea water to be delivered from the urea water pump toward the urea water injection valve flows, establishes a connection between the urea water pump and the urea water injection valve (Patent Document 1).